Motion picture cameras require a mechanism which advances the film past the camera shutter in an orderly fashion. In the case of most cameras, the film advancement mechanism comprises one or more claws which engage evenly spaced perforations located in the edges of the film and propel the film in a given direction. The advancement of the film occurs intermittently in a cyclical fashion, which as a general industry standard in cinematography occurs at 24 frames per second.
The film advancement cycle consists of a "pull down period" during which the film is advanced one frame, and a "dwell period" during which the film is stationery and is exposed when the camera shutter opens. During the pull down period, when the film perforation is engaged by the claws, the film is accelerated, decelerated, stopped, and finally the claws are disengaged from the film perforation.
In order to maximize the exposure time, and for high speed filming, the pull down period should be as short as possible. This requires that the film be advanced at a high rate of speed during the pull down period, which translates into high speeds of engagement and disengagement and high rates of acceleration and deceleration.
"Sawing" of the film perforation may occur during engagement and disengagement of the claw with the perforation. Sawing is due to lateral movement of the claw across the edge of a film perforation which causes wear on the perforation edges and ultimately damage to the film. The greater the speed of engagement and disengagement, the greater the lateral movement and the greater the sawing tendency. It is therefore desirable that engagement and disengagement of the claw with the film occur as smoothly and gently as possible to minimize sawing tendencies.
There is a further requirement that the film indexing from one frame to the next be as accurate and as consistent as possible. In order to achieve accurate indexing the claw should fit tightly in the film perforation in order to minimize film drift due to play between the claw and the perforation.
A number of prior art film advancement devices use reciprocating claw movements to advance the film during the pull down period. This type of device is exemplified in Borberg U.S. Pat. No. 3,249,274, Bach U.S. Pat. No. 3,229,879, and in Jacobsen U.S. Pat. No. 3,618,837. In the Bach device there is a substantial amount of sawing of the claw with the film due to the large lateral movement of the claw over the film perforation. In the Borberg device the claws do not decelerate the film; in fact the film impinges on the claws ejecting them and resulting in sawing action and film drift.
Additionally these devices tend to be inherently unbalanced causing unwanted vibration levels and being very complicated as evidenced by the complexity and number of parts required in the Jacobsen device.
There is accordingly a need for a simple, inexpensive film advancement device which minimizes the sawing of film perforations, the vibration levels, and the duration of the pull down period. There is a further need for a device in which the pull down period can be finely predetermined to ensure gentle engagement, smooth acceleration, smooth deceleration and gentle disengagement of the film.